1. Field of the Invention
This invention relates to electrostatic discharge (ESD) protection for circuits which employ bipolar transistors, and more particularly for circuits having a bipolar differential input.
2. Description of the Related Art
ESD events that occur differentially across the inputs of an unprotected bipolar differential input stage can produce currents that may permanently damage reverse biased base-emitter transistor junctions in the circuit. A common way to protect circuits such as comparators and operational amplifiers from ESD employs one or more diode-connected bipolar transistors at the node(s) to be protected. When an ESD event occurs at the protected node the breakdown voltage of the diode-connected transistor is exceeded, causing the transistor to conduct and draw the ESD current away from the protected circuit. One such protection scheme, disclosed in U.S. Pat. No. 5,637,901, protects an ingrated circuit (IC) from ESD spikes introduced at external terminal points. Since the diode-connected transistor breakdown voltage is almost always larger then the input differential breakdown voltage of an IC which includes a differential input stage, large resistors are connected in series with the inputs to limit the ESD currents flowing in the protected circuit. However, large resistors of this type degrade the performance of high speed ICs, which are intended to have propagation delays of less than 50 nanoseconds (nS).
The LT1016 comparator by Linear Technology Corporation has its differential input pins protected by connecting back-to-back diode-connected transistors in series across the circuit inputs. To be effective, the reverse bias emitter-base breakdown voltage of each diode-connected protection transistor must be lower than the reverse bias emitter-base breakdown voltages of the protected IC input stage, which may lead to a requirement for special fabrication processing. In addition, the diode-connected transistors are susceptible to the same degradation as that from which the input stage is being protected.
The Maxim Corporation MAX900/901 Voltage comparator employs diode-connected transistors in series with each differential input so that the protected input differential breakdown voltage includes the emitter-collector breakdown voltage of one diode-connected transistor and the base-collector voltage of the other. To work properly, additional diode-connected transistors are needed between each protected input and the positive and negative voltage supply lines to prevent differential ESD voltages from reaching the protected differential breakdown voltage. These additional diode-connected transistors limit the functional input voltage range of the protected device.